1. Field of the Invention
The present invention relates to transfer paper for electrophotography having coated layers on both sides used in an electrophotographic device, an electrostatic recorder or the like, which ensures a high paper feedability and conveyability and provides a high image quality in electrophotography.
2. Description of the Related Art
In the area of electrophotography, digitalization of color recording devices has recently made a remarkable progress, and efforts have been made to achieve a higher image quality. Along with this tendency, in the sector of on-demand publishing, color copying machines and color printers are employed more and more popularly because of the possibility to more easily publish a smaller number of books for a title. These printed matters are now manufactured by means of copying machines and printers.
Coated paper used generally for printing, known as art paper or coated paper, has been used when forming a high-quality image. In the area of electrophotography as well, coated paper is now an indispensable item for achieving a high image quality. It has however been discovered that, when using such coated paper in electrophotography, heat pressurization through fixing rollers may sometimes cause blisters.
A blistering phenomenon is caused, because of the low air permeability of usual coated paper for printing, by peeling of the paper layers upon thermal expansion of steam contained in the paper. In this respect, Japanese Patent Publication No. 5-82940 proposes a transfer sheet of paper for dry-type electrophotography based on a process of forming a coated layer by coating a coating material comprising a bonding agent containing a pigment having an average particle size of 1.5 μm or smaller and high oil adsorptiveness on both sides of a base sheet of paper, then applying a smoothing treatment, and adjusting the center line average roughness of the coated layer surface to 2.0 μm or under and the air permeability to 4,000 seconds or under. In the case of usual coated paper, however, the air permeability is on a level of about 6,000 seconds. As a result, the proposed technique is not applicable to ordinary coated paper.
A problem intrinsic to coated paper is that, for the purpose of achieving smoothness of the surface, it is the usual practice to apply a smoothing treatment known as a calendar treatment in the final stage during formation of a coated layer on the paper. In this smoothing treatment, however, a problem of the paper becoming easier to break is encountered. This leads to the necessity to increase the weight. This prevented paper having a weight from 60 to 130 g/m2 from being suitably employed for electrophotography. To solve this inconvenience, adoption of a method not applying an excessive pressure during formation of the coated layer was examined. Actually, it is tried to form a coated layer without decreasing stiffness of paper by use of a printer. When applying coating on such a printer, however, smoothness of paper is unavailable because of occurrence of print seams, but on the contrary, it is necessary to apply a pressure with a view to leveling the surface.